This is a revert of my previous commit to "improve" the declaration of

constant BIGNUMs. It turns out that this trips up different but equally
useful compiler warnings to -Wcast-qual, and so wasn't worth the ugliness
it created. (Thanks to Ulf for the forehead-slap.)

crypto/bn/bn_lib.c

@@ -887,11 +887,3 @@ int bn_cmp_part_words(const BN_ULONG *a, const BN_ULONG *b,
 		}
 	return bn_cmp_words(a,b,cl);
 	}
-
-/* See the comments surrounding BIGNUM_C in bn.h */
-#ifdef BN_DEBUG
-const BIGNUM *BIGNUM_CONST(const BIGNUM_C *bn)
-	{
-	return (const BIGNUM *)bn;
-	}
-#endif